Miniscule, invisible neutrino particles are ubiquitous but nearly impossible to catch. They fly though the Earth—and us—every moment, by the billions. They are the smallest known matter particles in the universe and were predicted to be massless. Strangely, though, they are not. The reason why is still a mystery, and scientists do not yet know how massive they are. Now a new experiment aiming to measure their mass directly has found they cannot weigh more than one electron volt (eV)—that is one 500,000th the mass of the electron, the next-lightest particle.

On September 13 the long-awaited results came from the Karlsruhe Tritium Neutrino (KATRIN) experiment, which recently began operating at the Karlsruhe Institute of Technology in Germany. The new measurement, based on just one month of data, cuts the possible upper limit of the neutrino mass in half, compared with the best previous estimate from a direct-measurement experiment. Neutrinos come in three types, called “flavors,” and the limit applies to the average of the three respective masses. From other particle physics experiments, scientists also know the average mass cannot be less than 0.02 eV. “The neutrino is now boxed in,” says Hamish Robertson, a KATRIN team member and a professor emeritus of physics at the University of Washington. “The exciting part is that we showed, in only a month, that we’ve already improved on the world knowledge that existed before.”

Figuring out the neutrino mass is important because these particles were not supposed to have any mass at all. The best theory physicists have for explaining fundamental particles, the Standard Model, had predicted they were weightless, so the 1998 discovery that they do have some small amount of mass came as a shock. “Neutrinos seem to have broken our understanding of what the Standard Model was supposed to be,” says KATRIN scientist Joseph Formaggio of the Massachusetts Institute of Technology. “It really does seem to hint at a larger theory.” Determining how much mass neutrinos have could help explain why they have it at all—and perhaps pave the way toward a deeper theory of physics that helps solve other mysteries as well.

KATRIN works by studying a natural radioactive process called beta decay, in which a neutron turns into a proton by releasing an electron and a neutrino. The experiment uses tritium gas—an unstable version of hydrogen with one proton and two neutrons (regular hydrogen has a proton and no neutrons). The extra neutrons in tritium are prone to beta decay; when this happens, the experiment isolates the ejected electrons and carefully measures their energies. Neutrinos themselves are too hard to catch, but scientists can calculate how much total energy the electron and the neutrino should share. Then, by subtracting the electrons’ energies, they can deduce the neutrinos’ energy—and therefore their mass. So far, KATRIN has not produced enough data to make a definitive mass measurement, but the fact that the neutrino mass is not yet obvious means it cannot be larger than 1 eV.

The true value is likely much smaller than that, according to cosmological data. Scientists can study how neutrinos’ gravitational pull affects the spread of galaxies throughout space—the larger neutrinos’ mass turns out to be, the greater the effect they must have had. “Cosmological bounds are complementary to these experimental results,” says Katherine Freese, a physicist at the University of Michigan, who works on astrophysical models to estimate neutrino mass and was not involved in the new experiment. “We, as a community, are really hemming in the value of neutrino masses from both sides at this point.” One recent cosmological analysis estimated that the smallest of the three neutrino masses cannot be larger than 0.086 eV—a lower limit even than the one KATRIN measured.

“There is indirect evidence that the neutrino masses are smaller than what KATRIN taught us last week,” says André de Gouvêa, a theoretical physicist at Northwestern University, who was also not involved in the measurement. “The indirect evidence does not replace what KATRIN can do, however, so the result in itself is very significant. Perhaps more important is that KATRIN demonstrated that things are working and that they appear to be on track to reach much further.”